The present invention relates to the measurement of soil compressibility and more particularly to an improved method and apparatus of insitu soil testing for determining soil compressibility at the base of a borehole.
The testing of soil compressibility at a construction site is important for determining the type of foundation which must be built to support a given building. Current methods available for determining the insitu characteristics of soil compressibility are basically of two types, plate bearing and pressuremeter.
In the plate bearing method, a plate is situated on the smoothed surface of the ground and its deflection is measured under incrementally increasing load. An acknowledged serious limitation of this test is that only near-surface soil contributes to settlement whereas the depth of influence of an actual foundation may be many times deeper. Therefore, if the soil changes with depth, the plate load test is not usable, or a pit must be prepared at great expense to conduct the test deep below the existing ground elevation.
The pressuremeters basically consist of a multi-chamber inflatable bag wherein lateral soil compressibility is monitored from a measured pressure versus volume relationship. The limitation of the pressuremeters is that only the lateral compressibility, not the vertical compressibility, is measured whereas placed on the soil or rock.
To overcome these limitations, two different types of tests have been proposed. The first is to conduct a type of plate test in the base of a borehole and the second involves laboratory testing of so-called "undisturbed" soil samples. But in each instance, the disturbance of the stress environment in the soil at the test site is reflected in the test results making them somewhat less than reliable.
Accordingly, a primary object of the present invention is to provide an improved method and apparatus of insitu soil testing for determining soil compressibility.
Another object is an insitu soil testing method and apparatus which produces a direct measure of soil compressibility in a vertical direction.
Another object is an insitu soil testing method and apparatus wherein tests may be conveniently conducted at the base of a borehole.
A related object is an insitu soil testing method and apparatus wherein tests may be conducted at any desired depth.
Another object is an insitu soil testing method and apparatus wherein the natural lateral stress environment of the soil at the base of a borehole is reconstituted prior to testing.
A specific object is an insitu soil testing method and apparatus wherein the test soil is laterally stabilized for substantially one-dimensional vertical compression testing.
Another object is an insitu soil testing method and apparatus providing for drainage of the test soil to enable measurement of the drained load deformation characteristics.
Finally, an object is an insitu soil testing method and apparatus which are simple, economical and efficient.